March 24, 1870] 
NA TORE 
541 
that its only communication with the North Atlantic basin— 
besides the circuitous passages leading into Hudson’s and 
Baffin’s Bays-—is the space which intervenes between the 
eastern coast of Greenland and the north-western portion of the 
Scandinavian peninsula. If, therefore, there be any such general 
interchange of Polar and Equatorial water as that for which we have 
argued, the Arctic current must flow through the deeper portions 
of this interspace, at the north of which lies Spitzbergen, whilst 
Iceland and the Faroes lie in the middle of its southerly expanse. 
Now in the channel that lies between Greenland and Iceland, 
the depth is such as to give a free passage to such a frigid stream ; 
but between Iceland and the Faroe Islands there is no depth so 
great as 300 fathoms at any part, except in a narrow channel at 
the south-east corner of Iceland ; so that an effectual barrier is 
thus interposed to any movement of frigid water at a depth 
exceeding this. A similar barrier is presented, not merely by 
the plateau on which the British Islands rest, but also by the 
bed of the North Sea ; the shallowness of which must give to 
such a movement a not less effectual check than would be 
afforded by an actual coast-line uniting the Shetland Islands and 
Norway. Consequently, it is obvious that a flow of ice-cold 
water, at a depth exceeding 300 fathoms from the surface, down 
the north-eastern portion of this interspace, can only find its way 
southwards through the deeper portion of the channel between 
the Faroe and Shetland !Islands; which will turn it into a 
W.S.W. direction between the Faroe Islands and the north of 
Scotland, and finally discharge such part of it as has not been 
neutralised by the opposing stream coming up from the south- 
west, into the great North Atlantic basin, where it will meet the 
Icelandic and Greenland currents, and unite with them in 
diffusing frigid waters through its deeper portion. In thus 
spreading itself, however, the frigid water will necessarily 
mingle with the mass of warmer water with which it meets, and 
will thus have its own temperature raised, whilst lowering the 
general temperature of that mass ; and hence it is that we do 
not find the temperature of even the greatest depths of the 
Atlantic basin nearly so low as that of the comparatively shallow 
channel which feeds it with Arctic water. 
It may be questioned, however, whether the whole body of 
Arctic water that finds its way through the channels just indi- 
cated, could alone maintain so considerable a reduction in the 
temperature of the enormous mass which lies below 1,000 
fathoms in the Atlantic basin; subject as this must be to con- 
tinual elevation by the surface-action of the sun on its southern 
portion. And as the few reliable observations on deep-sea tem- 
peratures under the equator indicate that even there a temperature 
not much above 32° prevails, it seems probable that part of the 
cooling effect is due to the extension of a flow of frigid water 
from the Antarctic Pole, even north of the Tropic of Cancer. 
Of such an extension there is evidence in the temperature-sound- 
ings recently taken in H.M.S. Hydra between Aden and Bombay, 
where the cooling influence could scarcely have been derived 
from any other source than the Antarctic area. * 
The unrestricted communication which exists between the 
Antarctic area and the great Southern ocean-basins would 
involve, if the doctrine of a general Oceanic circulation be 
admitted, a much more considerable interchange of waters 
between the Antarctic and Equatorial areas, than is possible in 
the Northern hemisphere. And of such a free interchange there 
seems adequate evidence ; for it is well known to navigators that 
there is a perceptible ‘‘set” of warm surface-water in all the 
Southern oceans towards the Antarctic Pole ; this ‘‘set” being 
so decided in one part of the Southern Indian Ocean, as to be 
compared by Captain Maury to the Gulf Stream of the North 
Atlantic.t Conversely, it would appear from the application 
of the necessary pressure-correction to the temperatures taken 
in Sir James Ross’s Antarctic expedition, the voyage of the 
Venus, &c., at depths greater than 1,000 fathoms, that the 
bottom-temperature of the deepest parts of the Southern Oceanic 
basin really approaches the freezing-point, or is even below it. 
And if the temperature of the deeper portion of the North 
Pacific Ocean should be found to exhibit a depression at all 
corresponding to that of the North Atlantic, it must be attributed 
entirely to the extension of this Antarctic flow; since the depth 
* The lowest temperature actually observed in these soundings was 36}°. 
The temperature of 33}° given in the previous discourse, as existing below 
1,800 fathoms, proves to have been only an estimate formed by Capt. Shortland 
under the idea that the rate of reduction observed at smaller depths would 
continue uniform to the bottom, which the serial soundings of the Porcupine 
prove to be by no means the case. 
+ ‘Physical Geography of the Sea,” §§ 748-750. 
of Behring’s Strait, as well as its breadth, is so small as to 
permit no body of Arctic water to issue through that channel. 
If further observations should substantiate the general diffusion 
of a temperature not much above the freezing-point over the 
deepest portions of the ocean-bed, even in Intertropical regions, 
as a result of a general deep movement of Polar waters towards 
the Equator, forming the complement of the surface-movement 
of Equatorial water towdrds the Poles, it is obvious that such 
diffusion must exert a very important influence on the distribution 
of animal life; and, in particular, that we may expect to meet 
with forms which have hitherto been reputed essentially Arctic, 
in the deep seas of even the Intertropical region, and again in 
the shallower water of the Antarctic area. Such, there is 
strong reason to believe, will prove the case. In his recent 
annual address as President of the Royal Society, Sir Edward 
Sabine cites observations on this point made by Sir James Ross 
in his Antarctic expedition, as confirmatory of the view enter- 
tained by that distinguished navigator, ‘‘that water of similar 
temperature to that of the Arctic and Antarctic seas exists in the 
depths of the intermediate ocean, and may have formed a 
channel for the dissemination of species.” The ‘‘ similar tem- 
perature” believed by Sir James Ross to have had this general 
prevalence, seems to have been 39°; whereas the observations 
made in the Porcupine expedition distinctly prove that a tempera- 
ture even below 30° may be conveyed by Polar streams far into 
the temperate zone, and that the general temperature of the 
deepest part of the North Atlantic sea-bed has more of a Polar 
character than he supposed. 
Again, the deep-sea dredgings of the Porcupine expedition have 
shown that many species of mollusks and crustacea previously 
supposed to be purely Arctic, range southwards in deep water as 
far as those dredgings extended—namely, to the northern extre- 
mity of the Bay of Biscay ; and it becomes a question of high 
interest whether an extension of the same mode of exploration 
would not bring them up from the abysses of even intertropical 
seas. 
Now, as there must have been deep seas at all geological 
epochs, and as the physical forces which maintain the oceanic 
circulation must have been in operation throughout, though 
modified in their local action by the particular distribution of 
land and water at each period, it is obvious that the presence of 
Arctic types of animal life in any marine formation cannot be 
accepted as furnishing evidence fer se of the general extension 
of glacial action into temperate or tropical regions. How far the 
doctrines now current on this point may need to be modified by 
the new facts now brought to bear on them, it will be for geo- 
logists to determine ; the question may be left in their hands 
with full assurance of a candid reception of the fresh evidence 
now adduced. 
The general results of the dredging operations carried on 
during the Porvcupize expedition will now be concisely stated. 
In the first place they show conclusively that there is no limit 
to the depth at which animal life may exist on the ocean-bed ; 
and that the types found at even the greatest depths may be not 
less elevated in character than those inhabiting shallower waters. 
It would even be premature yet to affirm that the higher types 
occur in less abundance and variety than at more moderate 
depths ; for it is by no means impossible that the use of the im- 
proved method of collection devised by Captain Calver, * which 
was employed with extraordinary success in the third cruise, may 
make as large an addition to our knowledge of the life of the 
sea-bottom explored by the dredge in the first and second cruises 
of the Porcupine, as it has done in the case of the cold area, 
where it revealed the astonishing richness of the bottom, which 
the Lightning dredgings of the previous year had led us to 
regard as comparatively barren, 
Secondly, they confirm our previous conclusion that tempera- 
ture exerts a much greater influence than pressure on the distri- 
bution of animal life. Not only have we found the same forms 
presenting themselves through an enormous vertical range—no 
amount of fluid pressure being incompatible with their existence 
—but we have also, by a more complete survey of the relations 
of the warm and cold areas, established the very marked differ- 
ence between the faunz of two contiguous portions of the sea- 
bed lying at the same depth, which was indicated by the Zivhé- 
ning dyedgings. It is remarkable, however, that this difference 
showed itself more in the crustaceans, echinoderms, sponges, 
* This consists inthe attachment of “‘hempen tangles” to the dredging 
apparatus, by which the floor of the ocean is swef/as well as scvafed. These 
tangles often came up loaded, when the dredge was empty. 
